[ES|QL] `JOIN` fields should follow field list autocomplete conventions

[drewdaemon]

Kibana version:
main

Join fields are inserted with a trailing space and a comma suggestion. This makes the user press the spacebar and leaves the comma hanging out in whitespace.

Screen.Recording.2025-03-13.at.4.38.51.PM.mov

We should instead follow the pattern we use for other field lists:

Screen.Recording.2025-03-13.at.4.41.06.PM.mov

This can be done using the handleFragment function like we do in KEEP.

[elasticmachine]

Pinging @elastic/kibana-esql (Team:ESQL)